Due to non-deterministic test failures in TestSceneBeatmapCarousel,
migrate the checkSelected helper step from AddAssert to AddUntilStep.
This adds more leniency for performance-related issues while still
checking the desired behaviour.
As the triangles intro relies on the audio track's clock advancing, we can't use it just yet (CI server has no audio device). This is a temporary workaround for that shortcoming.